Firing, in various forms has long been used as a means of disposal or removal of unwanted combustible waste. Volumes of smoke may be released if the process is uncontrolled, and environmental concerns have caused limitation and stringent regulation of such activities. However, The Environmental Protection Agency does find ecological profit in using properly controlled burn-off processes to reclaim materials for reuse or recycling, therefore such applications are now encouraged.
An oven, or primary chamber is fired and held at a given control temperature or "set point" by a thermostat. Those skilled in the furnace arts will add an afterburner to the oven. There, smoke and any gaseous components are combined with an excess volume of air in a lean, non-explosive mixture which is burned completely at elevated temperatures before being released into the air. This process differs from incineration in that the combustible materials are pyrolized in a low oxygen atmosphere in the primary chamber and pass to the afterburner as smoke and gases. The primary chamber thus operates at a relatively low temperature, protecting the reclaimable items. In this process there is a real risk of fire or explosion should the mixture/temperature profile exceed the lower explosive limit (LEL).
This process has heretofore been controlled in various ways, for example, as is disclosed by Kelly in U.S. Pat. No. 4,270,898. Kelly teaches the use of an oven water spray activated at a preset (1,700 F.) stack temperature, downstream from the afterburner. Water is sprayed in the primary chamber so that the concentration of combustible gases is reduced to a safe level. Mainord, U.S. Pat. No. 4,557,203, discloses a variation of the Kelly teaching wherein a modulated water spray is used while the primary burners stay on, even at oven control temperature, to better maintain an inert atmosphere. In any case, the operating temperature limit of 1,700 F. taught by Kelly is a severe limitation since elevated temperatures are necessary to achieve environmentally acceptable decomposition and oxidation of discharge gases.
Another control technique is disclosed by Koptis, et al. in U.S. Pat. No. 4,759,298, where he teaches control by adherence to a programmable oven temperature/time profile which is adapted to the product to be processed and the furnace characteristics. While such a system may allow safe operation, it also requires the operator to anticipate and evaluate all of the variables which enter into the combustion process; oven charge weight, composition, water content, BTU content, etc.
Each of these control systems assumes a constant set of conditions and responds in a rigid manner, where in fact, the conditions that lead to fire or explosion are widely variable. The LEL varies with oven temperature, gaseous mixture components, concentration and heat value, air flow, oven fuel flow and heat loss. As a consequence, performance is routinely compromised in quest of safety, resulting in either time consuming, inefficient operation or the discharge of incompletely processed materials. Even so, fires and explosions have been commonplace.
Accordingly, a first object of the present invention is to control the firing of combustible atmosphere furnaces by sensing and discouraging incipient ignition of the combustible atmosphere so as to avoid fire or explosion.
A second object is to control the firing of combustible atmosphere furnaces in a manner consistent with the elevated temperatures necessary to environmentally acceptable decomposition and oxidation of discharge gases.
A third object is to accomplish safe and effective combustible atmosphere furnace operation independent of operator judgement.